Restrepo, Filiatrault, and Christopoulos 2004
In current design approaches, structural systems are designed to respond beyond the elastic limit of the materials and to develop a ductile inelastic response mechanism that will dissipate energy in specific regions of the structural system. One method of achieving this is through the use of self-centering systems. The authors discuss applications of self-centering earthquake resistant structural systems that are intended to prevent excessive damage while emphasizing economic design. In this paper, after explanation of behavior of self-centering systems, early applications and recent development of the self-centering systems were presented.
Behavior of Self-Centering Systems
Optimum earthquake resisting systems limit the seismic forces that are transferred to the structure and provide additional damping characteristics that will return the structure to its original position after an earthquake and reduce the cumulative damage. Due to the characteristics of flag shaped hysteretic response of self-centering systems, the amount of energy transferred to the structure is reduced compared to that of the yielding system. Moreover, the system returns to a zero force and zero displacement point in every cycle and at the end of seismic loading.
Early Applications of Self-Centering Systems
The stepping rail bridge over the south Rangitikei River in New Zealand is an early application of a self-centering system. The self-centering system combined the rocking concept with energy dissipation devices. Base isolation allowed sideway rocking of concrete piers, while the weight of the bridge provided a re-centering capability.
Recent Development of Self-Centering Systems
Recent developments in self-centering systems for reinforced concrete structures, confined masonry walls, steel structures and bridge structures are explained by the authors. Resisting frame systems pre-stressed with partially unbonded tendons, precast beam column subassemblies, hybrid systems, and post tensioned split rocking wall systems are some example of systems to be applied to concrete structures. In one instance, wall panels are split to allow rocking of the individual panels about their bases. If the weight of the panel is not enough to re-center the structure, unbonded PT tendons can be installed. Grouting reinforcing bars into vertical ducts provides an energy dissipation mechanism.
In coupled wall systems, U-shaped rolled stainless steel plates are used. Observed damage was limited to the loss of two pieces of cover. When design drift was under 2%, there was no visible damage in the frame direction. Another example of a self-centering system uses unbonded tendons and conventional reinforcement for energy dissipation in reinforced concrete cantilever walls.
Engineers have also used a concrete coupled wall and cantilever walls with vertical joints. This acted as a hybrid system by using PT steel beams and unbonded PT tendons. PT steel beams have been shown to provide a significant restoring force to the walls, thus reducing residual lateral displacements.
In confined masonry walls, columns have been designed for strain control to ensure small shear distortions occur in wall panels, while the wall rocks at the foundation. In this case, energy dissipation devices can be installed at the wall toes.
A hybrid post tensioned connection has been developed in steel moment resisting framed structures. High strength steel strands are used in the connections. They attach to the beams by means of seat and top angles. Shear resistance is provided by friction at the beam-column interface and by the steel angles. In this system, steel angles are the only yielding elements and can be replaced after a major earthquake.
Finally, the authors discuss a self-centering base isolation system for bridge structures consisting of flat sliding bearings and precise positioning fluid dampers. This liquid spring damper is composed of a single column of a compressed fluid. The precise positioning mechanism uses a natural position, ensuing that it stays rigid before and after a shock.
Reference
Restrepo, J., Filiatrault J., and Christopoulos, C. (2004). “Development of Self-Centering Earthquake Resisting Systems,” Proceedings of the 13th World Conference on Earthquake Engineering, Vancouver, B.C., August 1-6.